Extending LilyPond: 2.8 Difficult tweaks

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2.8 Difficult tweaks

There are a few classes of difficult adjustments.

One type of difficult adjustment involves the appearance of spanner objects, such as slurs and ties. Usually, only one spanner object is created at a time, and it can be adjusted with the normal mechanism. However, occasionally a spanner crosses a line break. When this happens, the object is cloned. A separate object is created for every system in which the spanner appears. The new objects are clones of the original object and inherit all properties, including \overrides.

In other words, an \override always affects all pieces of a broken spanner. To change only one part of a spanner at a line break, it is necessary to hook into the formatting process. The after-line-breaking callback contains the Scheme procedure that is called after the line breaks have been determined and layout objects have been split over different systems.

In the following example, we define a procedure my-callback. This procedure

determines if the spanner has been split across line breaks
if yes, retrieves all the split objects
checks if this grob is the last of the split objects
if yes, it sets extra-offset.

This procedure is installed into Tie, so the last part of the broken tie is repositioned.

#(define (my-callback grob)
   (let* (
          ;; have we been split?
          (orig (ly:grob-original grob))

          ;; if yes, get the split pieces (our siblings)
          (siblings (if (ly:grob? orig)
                        (ly:spanner-broken-into orig)
                        '())))

     (if (and (>= (length siblings) 2)
              (eq? (car (last-pair siblings)) grob))
         (ly:grob-set-property! grob 'extra-offset '(1 . -4)))))

\relative {
  \override Tie.after-line-breaking =
  #my-callback
  c''1 ~ \break
  c2 ~ 2
}

When applying this trick, the new after-line-breaking callback should also call the old one, if such a default exists. For example, if using this with Hairpin, ly:spanner::kill-zero-spanned-time should also be called.

Some objects cannot be changed with \override for technical reasons. Examples of those are NonMusicalPaperColumn and PaperColumn. They can be changed with the \overrideProperty function, which works similar to \once \override, but uses a different syntax.
```
\overrideProperty
Score.NonMusicalPaperColumn       % Grob name
  . line-break-system-details     % Property name
  . next-padding                  % Optional subproperty name
  #20                             % Value
```
Note, however, that \override, applied to NonMusicalPaperColumn and PaperColumn, still works as expected within \context blocks.

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2.8 Difficult tweaks

LilyPond — Extending v2.21.4 (development-branch).